The invention relates to a paste composition useful for the production of electrically conductive and solderable structures generally secured to a non-conductive substrate.
A number of methods for the production of printed circuit boards are already known in which dispersions of organic polymers having a metal particle component, preferably silver particles, are deposited onto a non-conductive substrate, such as, hard paper or aluminum oxide, by spraying, drawing or screen printing processes. The coated substrate is cured by a subsequent thermal treatment. Electrical structures of this nature, which contain highly cross-linked and therefore thermosetting polymers, such as, epoxides or polyphenols as binding agents, are also distinguished by a good adhesiveness with regard to the metal particles dispersed therein. The problem with electrical structures of this type, however, resides with the electrical contacts which are not solderable.
One known method for the production of printed circuits utilizes a heat hardenable or heat-deformable plastic as the substrate material for the conductive component. This substrate is a mixture of insulating material and minute copper oxide particles on its outer surface, i.e., on the portions of the outer surface which are to be provided with conductive paths. The particles become rigidly connected with the insulating material during curing. The copper oxide particles on the surface are hereafter reduced to copper by sulphuric acid, phosphoric acid, etc. The surface is then rinsed and placed in an electroless bath in which copper or another suitable metal is built-up. The weight ratio of the copper oxide particles fluctuates between 0.25 and 80%. If the copper oxide content is near the lower value, it may be necessary to mechanically brush or scrape the surface in order to expose the particles near the surface to acid treatment. Without this additional preliminary handling the separation would require several hours. The acid treated substrate must immediately be additionally processed to avoid oxidation of the metallic copper which is exposed to the atmosphere. In electrical structures of this type the electrical capacity is determined exclusively by the thickness of the copper layer. To maintain good conductivity, the copper layer thickness must be relatively large. Building a relatively large copper layer from an electroless or currentless coppering bath, however, is a time consuming process even without the aforementioned additional preliminary handling. Another disadvantage of the known process resides in the fact that the adhesiveness of an electroless deposited copper layer generally decreases with increasing layer thickness, resulting in decreased reliability at a soldering point.